1. Technical Field
The present invention relates to a magnetic resonance imaging (MRI) apparatus in which an arbitrary tomographic image is obtained utilizing a nuclear magnetic resonance (NMR) phenomenon, and it particularly relates to the MRI apparatus by which an image quality therefor is improved.
2. Background Art
In recent years of a medical diagnostic apparatus being progressively developed, a magnetic resonance imaging (MRI) apparatus has been widely used. When a tomographic image of a biological body to be examined is imaged by the MRI apparatus, brightness of the tomographic image obtained by the MRI apparatus is generally not uniform even when the body is made of homogeneous material. This nonuniformity is often called a density inhomogeneity. A primary reason for the occurrence of the density inhomogeneity is nonuniformity of the high-frequency magnetic field (RF field inhomogeneity) generated by a radiofrequency (RF) coil. Due to the difference in the shape of the conductive pattern of the RF coil and the distribution of electric current flowing through the conductive pattern of the RF coil, both intensity and direction of transmit-receive sensitivity caused by the RF coil differ in three-dimensional spatial position. In other words, a high-frequency magnetic field vector (RF magnetic field vector) differs at each position.
Now, when the RF magnetic field produced by the RF coil is inhomogeneous in a space where there is placed the biological body to be examined, the magnetic field generated by a transmitting coil is spatially inhomogeneous. Thus, the RF magnetic field vector differs at each location inside the biological body. As a result, a flip angle differs at each location, where the flip angle indicates an amount of rotation of macroscopic magnetization vector produced by an RF pulse with respect to the direction of a static magnetic field.
Therefore, this non-uniform spatial distribution of the RF magnetic field vector intensity directly reflects on the intensity of receive signals. Accordingly, the inhomogeneity in the signal intensity is caused.
As another reason for the density inhomogeneity, there is a state in which the RF magnetic field is propagated in the biological body. This is due to a change in electromagnetic wave caused by interaction between the biological body and the electromagnetic wave. More specifically, the density inhomogeneity is caused by many factors, such as the dielectric constant (relative permittivity) and conductivity of the biological body, and distribution therefor and the boundary conditions determined by the shape of the biological body itself.
As for a general characteristic in the electromagnetic wave, energy thereof is consumed due to an eddy current generated in conductive material when the electromagnetic wave passes through the conductive material. Then, the wavelength thereof varies and there occurs a reflection of the wave due to the dielectric constant. That is, there occurs the reflection or refraction. These phenomena are also caused by the shape of the biological body. Owing to these above described compound effects, the RF magnetic field distribution inside the biological body becomes inhomogeneous depending on the shape, dielectric constant and conductivity of the body, even if the RF coil used produces a uniform (homogeneous) RF magnetic field. The unevenness of the brightness and darkness in the MRI images worsens as the magnitude of the static magnetic field becomes greater and as the Larmor frequency becomes high. Here, the Larmor frequency corresponds to a resonant frequency .omega..sub.o defined by the Larmor equation EQU .omega..sub.o =2.pi.f.sub.o.
In order to suppress the density nonuniformity (inhomogeneity), there have been proposed the following methods in the conventional practice.
When the uniformity of the RF magnetic field produced by the RF coil is not sufficiently obtained, the brightness of the material which is homogeneous on the image is made uniform by controlling a high-frequency power (RF power) of the transmitter RF coil. For example, in a case of imaging a fat layer of the stomach, even though the fat layer is of the same homogeneous material, there occurs a case where some portion of the fat layer is bright while another is dark. Thus, in order to alleviate this disadvantage, the power supplied to the transmitter coil is controlled. However, since the power is controlled, there is a strong chance that the quality of a thus obtained image is deteriorated. In other words, in this method, only the control concerning the RF magnetic field for the transmitter RF coil is performed while no control is carried out at the time of receiving such data by the receive coil.
As another conventional method, it is considered that a spatial distribution of the electromagnetic field of the RF coil is changed by varying the capacitance of distributed capacitors provided in a wire pattern of the RF coil (receive coil). However, in reality, it is difficult to change the capacitance distribution. Moreover, there can not be obtained a desirable effect by so doing.